Vitamin D plays a dual role as both a vitamin and a hormone that stimulates the body to absorb calcium. Vitamin D, calcium, and phosphorus are needed to keep bones strong. Without vitamin D even a diet adequate in calcium will not help as the body needs vitamin D to absorb calcium. People with low vitamin D levels tend to have soft, thin bones, predisposing them to fractures, especially hip fractures. Soft bones can also cause the nagging and unremitting bone pain that many older persons suffer from.
Vitamin D supplementation is important because it's hard to get enough during long, dark, northern winters. The elderly have more difficulty synthesizing vitamin D from sunlight, some medications interfere with vitamin D absorption and absorbing calcium which will strengthen bones and prevent fractures.
Vitamin D may be useful in resisting some forms of cancer as well. Breast tissue has receptors for the vitamin D hormone. When cultured outside the body, breast cancer cells do not reproduce as readily and in fact become more normal (differentiate) in the presence of vitamin D. These studies are interpreted as evidence that higher vitamin D levels have an anti-cancer effect on breast cancer cells. There are similar reports concerning prostate cancer and cancer of the large intestine.
Vitamin D3 (cholecalciferol from animal source) is the form found in fish oils and eggs, and is produced by sunlight on human skin. Natural Factors vitamin D3 is derived from sheep wool lanolin.